At the heart of photo-voltaic power systems are photo-voltaic cells that utilize semiconductor junctions. In contrast to other more traditional electrical power generators, photo-voltaic cells are very non-linear devices. Specifically, the output power delivered by a photo-voltaic cell is a very non-linear function of the load resistor applied to the cell. To maximize the power generated by a photo-voltaic cell or a photo-voltaic panel, a complex electronic circuit is needed to optimize the receiver load connected to the cell or panel. In most systems, this circuit is contained within a DC-to-AC inverter that connects the photo-voltaic cells or panels (which produces DC power) to the electrical grid (which delivers AC power). This circuit is often referred to as a Maximum Power Point (MPP) tracking circuit. Effectively, an MPP tracking circuit optimizes the input impedance of the inverter to operate at the maximum power point of the attached solar panels. An exemplary MPP tracking circuit for solar cell panel applications is described in U.S. Patent Application Publication Number 2013/0063117 A1 filed by Ki Su Lee, entitled “Maximum Power Point Tracking Method,” the contents of which are incorporated by reference as if fully set forth herein.
Inverters with a MPP tracking circuit often connect a large number of photo-voltaic panels to the electrical grid. The cost of the inverter is a significant fraction of the overall cost of the photo-voltaic system, and large inverters (from 10 s of kW to MW power) typically carry a lower cost-per-unit power than small inverters. Therefore the MPP tracking for power optimization is done on a large array of photo-voltaic panels in order to amortize the cost, and not on each photo-voltaic panel individually. However, since photo-voltaic panels are different from each other due to manufacturing variations, or when partial shading occurs, loading of individual photo-voltaic panels is not optimized and power production for each photo-voltaic panel is lower than what it could potentially produce. Power loss can reach 10% or more, depending on panel type, aging, and sun conditions.
Power optimization at the level of single panels does exist, either in the form of micro-inverters, or of power equalizers—both of which contain some MPP tracking electronics. However, owing to the way they operate, micro-inverters and power equalizers add substantial costs to the overall photo-voltaic system.
Therefore, effective and low cost techniques for power optimization at the level of single photo-voltaic panels would be desirable.